Scanning systems in modern day apparatus are becoming more accurate on the one hand but more complicated and expensive on the other hand. In certain raster scanning optical image systems, a collimated source of light, as from a laser and associated optical lenses, impinges on a rotating polygon scanner, which by the rotational movement thereof causes the reflected light to revolve about an axis near the center of rotation of the rotating polygon. This reflected light can be utilized to scan a document at the input end of an imaging system or can be used to impinge upon a photosensitive medium, such as a xerographic drum in the output mode. Many systems have been disclosed in the art to overcome various optical and other distortions in the optical flying spot imaging system.
For example, in U.S. Pat. No. 4,247,160, entitled "Scanner with Reflective Pyramid Error Compensation", issued to the same inventor and assignee as the present application, a laser beam scanner is disclosed having a positive cylinder mirror located between the polygon scanner and the photosensitive surface. The positive cylinder mirror, which has power in the cross scan plane but no power in the scan plane itself, minimizes scan to scan spot position errors due to angular alignment errors between adjacent facets of the polygon scanner without introducing significant cross scan field curvature. Further, this cylinder lens can be used to correct the effects of wobble introduced into the system by inaccuracies in the bearings, utilized to impart motion between the motor and the rotating polygon, and to the grinding or polishing of the actual facets on the faces of the rotating polygon itself. The wobble effects cause an uneven raster scan which produces irregular locations of the output scan lines whether utilized in an input or output fashion.
Canon Kabushiki Kaisha of Tokyo has eliminated the need for wobble correction by improving the tilt accuracy of the polygon facets to five arc-seconds, and by reducing the distance from the facet to the photoreceptor. See Minami, S. and Minoura, K., Proc. SPIE Optical Systems Engineering, Vol. 193, pp. 202-208 (1979). Since wobble is an angular error, reducing this distance reduces the lateral displacement due to wobble. The distance is reduced by decreasing the number of facets, thereby increasing the scan angle for each facet, and decreasing the lens focal length for a given scan. In this paper, the writers indicate the uses of an eight-facet polygon.
Eliminating cylinder lenses (1) removes the need to independently focus the scan and cross-scan planes, thus reducing alignment time, (2) reduces the numnber of lens elements, and (3) uses only rotationally symmetric (spherical) lenses which are cheaper to make than cylinders, greatly cutting fabrication and assembly costs. In addition, the short polygon-photoreceptor distance that Canon uses virtually eliminates the need for folding mirrors, and an eight-facet polygon is cheaper than an 18 facet polygon. By increasing the cost, or paying more for the use and manufacture of the polygon, the cost of the rest of the scanner system is reduced.
Workers at AGFA-Gevaert used double reflection at the facet to cancel wobble. After reflection from a facet, the beam is returned to the same facet by an arrangement of mirrors, for a second reflection. See Meeussen, et al, U.S. Pat. No. 3,897,132, assigned to AGFA-Gevaert. If there are an even number of reflections between facet reflections, wobble introduced at the first reflection is cancelled at the second. The cancellation is exact at the center of scan, where the wobble angle at the second reflection has the same value as the wobble angle at the first reflection. As the facet is rotated from the center of scan, the wobble angle at the second reflection is less than at the first, and cancellation is no longer exact. But over a small polygon rotation angle, wobble correction is nearly exact.
The Meeussen patent defines wobble as a deviation of the facet from parallelism with the axis of rotation. Thus, Meeussen's facet must be parallel to the axis and the patent does not consider draft angles. When the beam is offset in the cross-scan direction with a zero-draft angle polygon, bow is inevitable. If the embodiment in the Meeusen patent is analyzed, it can be shown that wobble is well corrected over the scan, but bow is excessive. The ends of a nine inch scan line are displaced at least three quarters of an inch from the center of scan in the crossscan direction.
According to the present invention, a polygon system for correcting wobble by double reflection from the active facet without bow is disclosd. Instead of the impinging light source being transverse to the axis of rotation of a rotating polygon, this rotating polygon having facets essentially parallel to the axis of rotation, there is disclosed a light source impinging upon a rotating polygon with the facets having the predetermined draft angle wherein by two further reflections, the light is then reimpinged upon the same facet, wobble is similarly corrected, but now with no effects of bow.